根据下列文章,请回答题。
　　Read the following text. Choose the best word(s) for each numbered blank and mark A． B．C．or D on ANSWER SHEET1.
　　In the past,the Park Service focused on making the big scenic parks more_______ 21 and comfortable for tourists. Roads were paved to allow "windshield visitors" to experience the grandeur of nature without leaving their cars, and a _____ 22 number of hotels and grocery stores were permitted to open _____23 the park boundaries.
　　Now this trend is changing. Plans have been made to _____24 the parks to their natural condition as much as possible. The objective of such a move would be to secure the preservation of the parks for future generations, _____ 25 allowing present-day visitors to experience pure wilderness, _____26 from any obvious signs of civilization -- an opportunity which is quickly disappearing in the twentieth century. _____27 plans call only _____28 a reduction in the number of cars _____ 29 into the parks each day, but _____ 30 ,tourists may have to leave their cars at the gates and then either visit the park on foot _____31 use park _____ 32 . _____33 , stores and hotels may no longer be allowed within park boundaries and even the number of campgrounds may be restricted.
　　Denali National Park in Alaska serves as an excellent _____34 for this new type of park, one which has been changed only slightly from its _____ 35 state. There is only one road, unpaved in _____ 36 , which cross _____ 37 Denali. As car traffic is strictly limited, many visitors experience the magnificent _____38 and wildlife from a park bus. There are no hotels or stores and only seven campgrounds within Denali's 3,000 square miles. This _____ 39 isolation offers backpackers,canoeists, and other sport enthusiasts a _____40 .physical and psychological challenge.